Geofoam Geosynthetic:
Past, Present, and Future
Professor of Civil Engineering
Manhattan College,
Bronx, New York, NY, USA
e-mail
INVITED PAPER
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Geofoam Geosynthetic: Professor of Civil Engineering |
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INVITED PAPER |
ABSTRACT
Polymeric (plastic) and glass foams have been used
in geotechnical applications since at least the 1960s as thermal
insulation, lightweight fill, and for many other functions. Since
1992, any type of foam used in a geotechnical application has
been considered to be a geosynthetic product called "geofoam."
This new terminology coincided with a rapid expansion worldwide
in the knowledge and use of foams in geotechnical applications.
The primary focus of this paper is to list geofoam research and
development needs to support the continued growth of geofoam technology.
A brief overview of geofoam materials, and past and current uses
of geofoam is also included to provide background information
for understanding these research and development needs.
INTRODUCTION
What is Geofoam?
Because the term geofoam has only been used since 1992, there is still some confusion as to its definition. Quite simply, geofoam is the generic name for any foam material used in a geotechnical (on- or in-ground) application. Geofoam is now recognized worldwide as a geosynthetic product category in the same sense as geotextiles, geomembranes, geogrids, etc.
Scope of Paper
This first issue of The Electronic Journal of Geotechnical Engineering is an important step in bridging between the past and future forms of professional/technical communications media. In keeping with this theme, the goal of this paper is to explore the future research and development needs of geofoam geosynthetic technology. However, the past and present are reviewed as they provide the necessary basis for planning for the future.
The specific content of the sections of this paper are as follows:
Primer: Although geofoams have been used at least since the 1960s, they are still a relatively new material to many geotechnical engineers, especially in certain countries such as the U.S.A. Therefore, a brief primer on geofoams is given for the benefit of those many engineers just learning about geofoam.
Past: Geofoam evolution and past usage is summarized to provide an understanding of current practice. Present: A summary of the present state of geofoam usage is presented to provide a basis for suggested future research and development.
Future: This is the primary focus of this paper. Specific suggestions for developing the various aspects and applications of geofoam technology are given.
The first four sections represent a heavily abridged version of what can be found in Horvath (1995). Therefore, interested readers are directed to that reference for additional information on these topics. The last section represents an updated version of what appears in Horvath (1995).
GEOFOAM: A BRIEF PRIMER
Functions
Because it is a geosynthetic, the correct way to design with geofoam is to "design by function," a process that has proven effective with other types of geosynthetics (Koerner 1994). Design by function is a design philosophy rather than a particular methodology. It simply means that the end user (design professional (engineer, architect), specifier, builder, etc.) making the decisions as to what geosynthetic product(s) are to be used in a particular application first decides which of several geosynthetic functions are required to be provided, then selects the geosynthetic product(s) that will satisfy these needs most cost effectively.
There are several aspects regarding the functions provided by geofoam that are of particular interest:
Materials
The current definition of geofoam as proposed in Horvath (1995) is any manufactured material created by some expansion process that results in a material with a texture of numerous, closed, gas-filled cells. The cell walls are solid although permeable to gases. Most geofoam materials are polymeric (plastic) but glass foam (cellular glass) has been and is also used. Although gases (called blowing agents) other than air are typically used in manufacturing geofoams, with time (which can vary widely depending on the geofoam material, a fact that many fail to consider properly when designing) the cells eventually become filled with air.
There is some ongoing discussion that the current definition of geofoam should be broadened to include open-cell materials which are polymeric and created by some type of extrusion process (Perrier 1996). However, this has not yet been resolved and such open-cell materials are not considered further in this paper.
Polymeric materials have always dominated the geofoam market. Several different polymers have been tried in geofoam applications but the one used most commonly by far is polystyrene. There are two ways to manufacture polystyrene foam:
By the two-stage molded-bead process which produces molded expanded polystyrene or, as it is more commonly called, expanded polystyrene (EPS). Because the individual beads (produced during the first stage) can generally been seen in EPS it is sometimes referred to colloquially as "beadboard" although the EPS industry seems to deprecate this practice.
By a continuous extrusion process which produces extruded expanded polystyrene or simply extruded polystyrene (XPS). A simple way to identify XPS geofoam is that it is always colored for proprietary marketing purposes (EPS geofoam is generally left in its natural white color). For example, in the U.S.A. XPS geofoam is colored either blue, green, pink, or yellow depending on the manufacturer.
It is important to note that polymeric foams should never be referred to generically as "styrofoam," a practice that, unfortunately, appears to be widespread in the U.S.A. and possibly elsewhere. Styrofoam is actually the registered trademark of a brand of blue-colored XPS that is available in the U.S.A. and possibly elsewhere. As always, correct generic terms such as EPS and XPS should be used in geosynthetics unless a particular product is indeed meant.
Because polystyrene foams have and do dominate the geofoam market, the remainder of this paper will focus on them.
Products
Expanded Polystyrene (EPS)
There are two primary methods for molding EPS:
There is a variation on block molding called slab molding in which relatively thin panels are produced directly, sometimes with a custom shape so that it is actually a slab-shape hybrid. Such products for geofoam applications are relatively rare (Canada is one country where they are relatively common) because of the highly specialized molding equipment required. In addition, there are other niche geofoam materials such as glued or molded polystyrene porous block and elasticized EPS block that are described in Horvath (1995).
Geocomposite products that utilize EPS as a component are becoming increasingly common. One example is the Geoinclusion which is available in North America. This product uses a panel of elasticized-EPS-block geofoam as its primary component plus a drainage geocomposite that is factory laminated to one face of the panel.
Extruded Polystyrene (XPS)
XPS is produced primarily in plank-shaped pieces. It is possible to custom-extrude a particular shape but the distinction between planks and shapes in geofoam terminology is not done (at least to date) as it is for EPS.
Durability
Durability of geosynthetics in general has been a subject of great interest in recent years. Overall, the durability of EPS and XPS geofoams is excellent. There is experience dating back to the 1960s to support this. Typically, the only concern with EPS and XPS geofoams is that they be protected from gasoline and similar petroleum-hydrocarbon liquids with a geomembrane or similar barrier in applications where there is a potential for a fuel spill (e.g., road embankments).
In addition, in some applications (thermal insulation around the below-ground space of buildings is one in particular) there have been problems with infestation by certain burrowing insects (termites, carpenter ants). It appears that an effective passive treatment against potential insect infestation has been developed for EPS-block but not XPS.
GEOFOAM: THE PAST
Overview
As noted previously, foams have been used successfully in geotechnical applications since at least the mid 1960s. However, most engineers are unaware of this history of usage that is longer than for virtually all other geosynthetic products so a brief review of geofoam history is presented in this section. This section is also used to introduce and describe the geosynthetic functions that geofoam can provide.
Functions and Their Applications
Thermal Insulation
EPS and XPS were invented circa 1950 primarily to provide thermal insulation. Foams in general are very efficient thermal insulators because they are approximately 98% to 99% gas by volume (the gas trapped in the cells of foams is air, at least in the long term) and gases are typically very efficient thermal insulators. Therefore, it is perhaps not surprising that the first known application of what we now call geofoam was as thermal insulation of roads, railways, and airfield pavements (to prevent or at least reduce seasonal frost heaving or retard thawing in permafrost areas); the below-ground portions of buildings (to reduce seasonal heating requirements); and beneath on-grade storage tanks containing cold liquids (one of the few applications where glass foam is used almost exclusively) beginning in the mid 1960s.
Each of these applications was successful in its original goals. Unfortunately, unexpected problems developed with pavement insulation because of a phenomenon called "differential icing." Simply stated, this means that an insulated pavement develops surface icing (a potential safety hazard) sooner and/or more extensively than an adjacent uninsulated pavement. This led many early users of insulated pavement, especially government agencies (typically state departments of transportation in the U.S.A.), to limit or ban outright the future use of insulated pavements. This is unfortunate as insulated pavements can be quite effective in reducing pavement damage due to frost heaving. Extensive research into the causes of differential icing was subsequently conducted in Scandinavia, especially at the Norwegian Road Research Laboratory (NRRL). This led to the development of design strategies that eliminate or at least minimize differential icing.
Lightweight Fill
Geofoams, especially polymeric ones, are unique materials in that they have a density that is only about 1% to 2% of the density of soil and rock yet are sufficiently strong to support many types of loads encountered in geotechnical applications. Thus one of the earliest functions of geofoam that was developed was its use as a lightweight fill material (some have used the term ultralightweight which is arguably better but not used in general) in a wide variety of "earthworks." The general benefit of using geofoam as opposed to other materials in earthworks is the significantly reduced stresses on the underlying subgrade. This can have multiple benefits in terms of reduced settlements, increased stability, etc.
The exact origins of using geofoam as lightweight fill are contentious and, unfortunately, often acrimonious. However, what can be stated without dispute is that the use of geofoams as lightweight fill began circa the early 1970s. While both EPS-block and XPS were tried initially, the economics were, and still are, clearly in favor of EPS block. Equally clear is that the credit for the systematic development of the use of EPS-block geofoam as lightweight fill belongs solely to the NRRL. The NRRL has also been very generous in sharing their knowledge with others without seeking any patent royalties, and the first international symposium (organized by the NRRL in 1985) on using EPS-block geofoam as lightweight fill was the catalyst for its use in other countries, especially Japan where significant additional research and development (especially into seismic loading) has been conducted in recent years.
It is interesting to note that the use of EPS-block geofoam as lightweight fill has tended to flourish in those countries such as Norway and Japan where an organization of some sort existed or was established to promote its use. On the other hand, in countries such as the U.S.A. where there was until recent years chronic indifference to geofoam technology the use of geofoam as lightweight fill has languished. This is pointed out because many engineers in countries such as the U.S.A. have an initial distrust of geofoam from the viewpoint of "how come it has been around so long but used so little?" Clearly, the lack of meaningful promotion and technology transfer is the root cause, not some hidden technical shortcoming.
Compressible Inclusion
One of the very useful aspects of EPS-block geofoam is that it can be manufactured over a range in densities. This is relevant because if EPS block is manufactured to certain quality standards then (and only then) can density be a useful index property in the same way that particle size of granular soils or Atterberg Limits of plastic soils are useful index properties of soils.
Beginning in the mid to late 1980s (as with so many things, the exact origin is difficult to identify), it was noted that some geofoam products (glued polystyrene porous block and EPS-block) could be used in applications where relatively large compressibility was desired. Examples of such applications are above pipes and culverts to induce vertical arching or behind non-yielding walls to induce lateral arching and the active earth pressure state. In each of these applications, the benefit of using geofoam for what is called the compressible-inclusion function is a significantly reduced load on a structure (pipe, culvert, retaining wall, etc.).
Drainage
Typically, geofoam materials have very low permeability for fluids (both gases and liquids). However, both EPS and XPS geofoam products can be factory cut or purposely shape molded to have a geometry such that they readily transmit fluids (especially ground water) along one face or side of the product. Recently, this has been extended to EPS-shape products intended to readily transmit ground-borne gases such as methane and radon.
In addition, there are geofoam materials that have an inherent permeability throughout their entire thickness. The most-common example is glued polystyrene porous block. This panel-shaped product uses expanded spheres of polystyrene that are glued into an open matrix. One face of the panel is typically covered with a geotextile which provides separation and filtration functions.
Damping
The inherent very low density yet significant stiffness of geofoam can be beneficial in reducing ground-borne, small-amplitude waves that produce noise or ground motion that may be disturbing to people and/or harmful to sensitive equipment. Typical sources of such vibrations are motor vehicles or trains. It should be noted that for vibrations of large amplitude, such as from earthquakes, where relatively large movement of the ground (i.e., soil particles) is involved, the benefit of using geofoam appears to derive more from the compressible-inclusion function rather than the damping function.
The first use of geofoam, specifically, EPS block, to reduce small-amplitude vibrations is unclear, but papers and magazine articles on the subject have been found as far back as the 1980s. Regardless of its origin, this is arguably the least studied and utilized geofoam functions to date.
Structural
This is the newest geofoam function and its exact definition is, consequently, still in the process of evolution. Included in this category are those applications where the geofoam is either serving as a structural element or some application that does not clearly fit into another functional category.
GEOFOAM: THE PRESENT
Overview
In this section, the status of present geofoam usage is summarized. This is to provide a basis for future activities which are suggested in the next section, the future. Again, a functional organization is useful for this purpose.
Functions and Their Applications
Thermal Insulation
There have not been any radical developments in applications involving this function in recent years. Rather, there have been evolutionary improvements and extensions of applications involving thermal insulation of the below-ground space of buildings. Examples include:
The use of products, especially made of EPS block or shape, to act as both concrete formwork for below-grade walls during construction and permanent thermal insulation afterward.
The us of the Frost Protected Shallow Foundation (FPSF) concept to allow significantly shallower footing embedment depths for structures without below-ground space.
With regard to insulated pavement, unfortunately it appears that many organizations (especially government agencies in the U.S.A.) that might use or approve use of insulated pavements have retained a permanent bias against this technology because of the initial problems with differential icing in the late 1960s/early 1970s. In addition, it appears the benefits of using insulated railway track systems has not been utilized at all in many countries (such as the U.S.A.) where it might prove cost effective.
Lightweight Fill
The use of EPS-block geofoam as lightweight fill represents a fairly mature application that, at long last, is spreading rapidly in countries such as the U.S.A. where the technology has languished for decades. However, further research and refinements into this application have been made in recent years. Among the more noteworthy are:
Ongoing research (primarily conducted in Japan)
to better understand the behavior of EPS-block geofoam fills under
seismic loading.
The increasing use of "geofoam walls."
These are applications, typically in what are called side-hill
fills, in which the exposed (downslope) face of the geofoam is
made vertical or near vertical as opposed to sloped. This allows
use of a considerably reduced volume of geofoam (a cost reduction)
as well as requires significantly less right of way, an important
consideration in hilly or mountainous terrain.
Compressible Inclusion
This is the geofoam function that has seen the greatest research interest in recent years. Rather than repeat the many activities in this area which are literally a paper in themselves, interested readers are directed to the following current publications:
An overview of this function and its numerous applications can be found in Horvath (1996a).
A detailed discussion of analytical methods for various applications can be found in Horvath (1996b).
Drainage
The most significant activity in this functional
area in recent years has been:
The development of EPS-shape geofoam products in
the U.K. specifically for ground-borne gas drainage.
The use of existing drainage products (specifically
glued polystyrene porous block) as part of the new Geoinclusion
product.
Damping
Little has been done in recent years to further develop this function. Documented case-history applications remain few in number.
Structural
This function has seen the greatest relative growth in recent years because it is the newest geofoam function. The primary product developed to date is the use of panels of EPS block as facing panels in what is essentially a mechanically stabilized earth (MSE) wall. This is the Tipform system which was developed in the U.K. Other products include the use of pieces of EPS as void formers in drilled-shaft construction, an application most exploited to date in the U.K..
GEOFOAM: THE FUTURE
Overview
Successful development and implementation into civil-engineering practice of any technology requires that a trilogy of related and coordinated activities occur more or less simultaneously. This trilogy can be likened to a three-legged stool. If any leg is missing or even shorter than the rest, then the stool is inherently unstable and unusable.
The components of this trilogy are:
Note that this trilogy is never completed, but just assumes a constant, repetitive cycle of growth.
In the following sections, a detailed discussion of the needs of each component of this trilogy is discussed.
Technology Transfer (Education)
The critical need to synthesize almost 30 years of geofoam usage worldwide was provided by the publication of Horvath (1995). This document effectively summarizes the state of knowledge, including an extensive bibliography, up to the time of the publication of this monograph (mid 1995).
In keeping with the futuristic venue of an electronic journal in which this manuscript is appearing, it is appropriate to think in a similar progressive manner as to how future developments in geofoam technology will be communicated to both end users and manufacturers as we enter the 21st century. Overall, the author's "vision" of this is that the primary medium for distributing information will be the Internet/World Wide Web (WWW) or some successor type of "real time" communication in which an interested party can access information essentially instantaneously on demand. Print and even CDROM media will play less and less of a role as time goes on.
With specific application to geofoam, there will be two components to this real-time communication:
Predictions of the future are notoriously short on specifics and the ones made here by the author are no exceptions to this. However, perhaps the most important fact to keep in mind is that one trend that has already developed on the WWW is that users expect free information. While many have tried to date to "make money" from the WWW, it is the author's opinion that it is difficult to do so directly, e.g., by charging to access a site. Therefore, it is proposed that rather than try to charge an access fee to read a design guide, for example, on the WWW, access might be free with development cost borne by industry who would recoup their investment through sales.
The author is fully aware of the practical problems associated with securing funding for widespread industry support of these proposed efforts, especially in the rather fragmented geofoam industry. However, the WWW was founded on the premise of free access and information. The author's perception is that to turn the WWW into a for-pay operation at this relatively late time will be difficult. This is supported by the author's observation, as unscientific as it may be, that attempts to charge for WWW services directly are generally unsuccessful. Therefore, it is the author's opinion that education in the geofoam industry should be supported by the relatively few raw-materials suppliers worldwide (this would be resin suppliers in the EPS industry) as the fewer the parties involved the relatively easier it will be to develop consensus support.
Technology Documentation (Standards)
This is arguably the weakest leg of the current technological trilogy of geofoam. It is especially true in countries such as the U.S.A. where geofoam standards are essentially non-existent which requires the interim use of ASTM standards that are not totally applicable to geofoam needs. This deficiency is not unlike the situation that existed early in the technology for other types of geosynthetics. While some geofoam standards development has occurred in a few countries, the fact that most geofoams, especially EPS block, are the same and are available worldwide suggests that geofoam standards activities might be better served by coordination through an international geosynthetics organization such as the well-established Geosynthetic Institute.
Technology Advancement (Research and Development)
Material Testing and Constitutive Models
This section will focus on needs for EPS block. Not only is it the most commonly used geofoam material, especially in load-bearing applications, but it is inherently generic. Needs for proprietary materials and products are considered beyond the scope of this paper.
Although the basic behavior of EPS block is well established after almost 50 years of testing and use, additional testing in selected areas of interest for geofoam applications is still required. Specific areas requiring attention include:
A study of stress-strain behavior, especially under creep conditions, for temperatures greater than those typically encountered in a laboratory environment (approximately +23C). The elevated temperatures selected for testing should reflect the upper range of average annual air temperatures found in the warmest climates of the Earth. This is particularly relevant because it is in such areas such as Southeast Asia where much of the growth of geofoam usage, particularly for lightweight fill applications, is likely to occur. It is the author's opinion that the current body of laboratory test data and case history experience (overwhelmingly in relatively cool climates) may not be wholly appropriate because the creep of polymeric geofoams is known to increase with increasing temperature.
A fundamental evaluation of the behavior under relaxation (stress reduction with time under constant strain) is required. To date, relaxation behavior has been inferred from mathematical manipulation of creep-test data but this needs verification using the results of explicit relaxation testing before it can be used with confidence in practice.
It is strongly suggested that that atmospheric (barometric) pressure be recorded during all future tests, especially those of extended duration such as creep and relaxation. Unpublished relaxation tests performed by the author suggest that the air pressure inside the cells of EPS lags atmospheric pressure changes. Thus depending on the relative pressure change, the EPS can appear to be temporarily stiffer or softer than some average value because of differential pressures across the cell walls. This phenomenon, which was encountered unexpectedly by the author, requires further study.
To make the best use of the extensive material testing performed to date and planned for the future, there needs to be a consistent program for developing constitutive (mathematical) models, first for EPS block and eventually for other geofoam materials. Such models should incorporate as a minimum stress, strain, time, and density as variables, with temperature to be added at a later date when sufficient data are available. This need is driven by the increased sophistication in geofoam analyses that has occurred in recent years, especially using the finite-element method. While sporadic attempts have been made at constitutive modeling, it would be arguably better if work were coordinated within a single framework.
Applications by Function
Thermal Insulation
The efficacy of EPS and XPS geofoam as thermal insulation is well established. What is needed are efforts in two broad areas:
Specific suggestions for each case are given subsequently.
With regard to underutilized applications, the classic
example is insulated pavements. It is clear that a new assessment
of the cost-effectiveness of insulated pavements is long overdue,
led by the current generation of geotechnical engineers who are
not biased by early problems with his technology. After all, pavements
that develop potholes as a result of frost heaving create their
own type of safety hazard as well create costs for pavement repair
(almost always unsuccessful in the long term) as well as for vehicle
damage. As noted previously, design strategies exist that will
largely overcome differential icing which was the primary reason
for abandoning pavement insulation originally.
There is no doubt that evaluation of insulated pavement systems would be greatly assisted by development of simplified but comprehensive models that allowed for cost optimization of systems while considering:
Pavement construction cost as reflected in the protection offered against frost heaving. When insulated pavements were first used, they were designed to provide full protection against frost penetration into the subgrade beneath the geofoam and concomitant frost heaving. Some later designs allowed partial frost penetration of the subgrade, primarily as an economy measure (thinner panels of geofoam were required).
Differential icing which is largely affected by how deep in the pavement section the geofoam is placed and the nature of the soil overlying the geofoam.
Cost per thermal resistivity ("R" value in U.S. terminology) per unit thickness. This is because the most-efficient geofoam material thermally may not be the most cost effective. This cost calculation should include a realistic allowance for long term water-absorption. All geofoam materials absorb ground water with time and this reduces their thermal resistivity so should be accounted for in design. As part of this, there should be a consideration of the cost effectiveness of using geomembranes and/or using geonets to reduce water absorption. Studies with residential geofoam thermal insulation have indicated that water absorption, at least for EPS, can be reduced to almost zero with the use of other geosynthetics which simply did not exist when insulated pavements were first tried.
Pavement life-cycle costs as reflected in the number of load cycles before pavement repair or replacement is required. This depends on both the deflection per load cycle as well as any heave/thaw damage if the design is for only partial insulation.
An excellent example of the framework for such a cost-based technical model (although certain requisite parameters such as differential icing were not included) is the study reported in Dore et al. (1995).
With regard to research and development for new applications, one of the ones that seems to be in some demand from engineers, at least in the U.S.A., is a relatively simple and practical method for designing insulation for buried water and sewerage utility lines in relatively cold climates.
Lightweight Fill
The efficacy of using EPS-block geofoam is well established. What is needed are efforts to develop procedures to make selection of cost-effective designs simpler in routine practice. Several areas where such efforts would be useful include:
A cost-based technical optimization procedure similar to that recommended for insulated pavements but to determine the optimum pavement system (including whether or not a reinforced-concrete slab is used) to be placed over the EPS blocks. This should include consideration of:
A simplified procedure for determining stress distributions through an EPS fill so that the lowest density blocks can be selected for each area of a fill based on an assumed maximum magnitude of allowable strains.
Development of standardized design details for facing systems (shotcrete, precast concrete panels, etc.) for geofoam walls.
Compressible Inclusion
This is the function requiring the greatest research because there is perhaps the greatest diversity of potential applications. Applications fall into two broad categories:
With regard to earth retaining structures, there
are numerous combinations of variables that require evaluation
and the development of analytical techniques suitable for routine
practice. Key variables are:
Research efforts should take advantage of the full spectrum of physical testing (geotechnical centrifuge, large-scale-model shake table, and instrumented retaining wall test facilities) as well as a parallel effort of numerical modeling using the finite-element method.
Drainage
There are significant potentials for using geofoam for this function if the geofoam is being used for one or more other functions. In general, geofoam products are not cost effective compared to other drainage geocomposites when only drainage is required.
Damping
There is significant potential growth for using this function, especially when the geofoam is being used for other functions. What is needed to make greater use of this function are analytical methods that are amenable for use in routine practice so that the benefit if using geofoam to dampen noise and small-amplitude ground vibrations can be estimated beforehand.
Structural
There appears to be significant potential for using geofoam panels as facing for MSE walls (in the way that precast concrete panels are used now) and as blocks in segmental retaining walls (SRWs) in that way that concrete blocks are used now. The benefit of this may be particularly useful under seismic loading where the very low density of EPS geofoam can be beneficial.
ACKNOWLEDGEMENT AND DEDICATION
The author is grateful to the those organizations and their personnel, too numerous to mention here individually (they are, however, given their well-deserved thanks in the Preface in Horvath (1995)), who, since 1988, have generously shared their knowledge related to geofoam materials and products with the author. The past and current successes of geofoam as a civil engineering material is due in large part to their foresight, efforts, and generosity.
REFERENCES
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